Self-injurious behavior (SIB) presents a serious problem in laboratory macaques that cannot be socially housed for scientific reasons. It is also a serious problem among significant numbers of institutionalized children and adults where it is often associated with different forms of brain dysfunction. In macaques, SIB is associated with conditions of social deprivation during early development. We have experienced limited success in reducing SIB in macaques by enhancing their environment with enrichment devices. Psychotropic drugs also help, but problems are associated with their use. Because sexual and aggressive behavioral problems in men have been treated with drugs containing progestin and androgenic profiles, we tested the efficacy of cyproterone acetate (CA) on reducing SIB in eight singly-housed, adult male rhesus macaques. The main findings were 1) the frequency of occurrence of SIB, and some other atypical behaviors, was significantly reduced after several weeks of CA treatment; 2) blood levels of testosterone were also significantly reduced, but 3) frequency of masturbation and sperm counts did not drop with the treatment (5-10 mg/kg/wk CA). We concluded that CA is a partially effective, long-term treatment (3 months) for adult male macaques whose behavior problems include SIB. However, in addition to CA, other interventions are needed to eliminate SIB. The findings suggest that SIB is modulated by progestins or androgens. The ability of CA to reduce SIB, but not the frequency of masturbation or sperm counts, has led to the hypothesis that its androgen-like action is sufficient to maintain sexual behavior, but its progestin-like action may decrease aggressive behavior. This may also indicate a separation of the neurophysiological mechanisms that control sexual and aggressive behavior. We conclude that CA would be useful in treatment of SIB, especially when it is desired to maintain reproductive capabilities in the subjects.